This invention relates generally to gas turbine engines and more particularly to film cooled combustor liners used in such engines.
A gas turbine engine includes a compressor that provides pressurized air to a combustor wherein the air is mixed with fuel and ignited for generating hot combustion gases. These gases flow downstream to one or more turbines that extract energy therefrom to power the compressor and provide useful work such as powering an aircraft in flight. Combustors used in aircraft engines typically include inner and outer combustor liners to protect the combustor and surrounding engine components from the intense heat generated by the combustion process. The combustor liners are cooled to meet life expectancy requirements.
Liner cooling is commonly provided by diverting a portion of the compressed air (which is relatively cool) and causing it to flow over the outer surfaces of the liners. In addition, a thin layer of cooling air is provided along the combustion side of the liners by directing cooling air flow through circumferentially disposed rows of cooling holes formed in the liners. This technique, referred to as film cooling, reduces the overall thermal load on the liners because the mass flow through the cooling holes dilutes the hot combustion gas next to the liner surfaces, and the flow through the holes provides convective cooling of the liner walls. In one known configuration, film cooling is accomplished by forming the liner with a plurality of integrally formed panel sections with a bump or nugget formed on the forward end of each panel section. A circumferentially disposed row of axially oriented cooling holes is formed in the nugget so that the thin film of cooling air is produced along the inner surface of the panel section.
While film cooling of combustor liners is generally quite effective, various conditions in gas turbine combustors can reduce the cooling film effectiveness in specific regions of the liners. For instance, due to various phenomena that commonly occur inside combustors, the heating load is not uniformly distributed about the liners. This leads to specific regions in the combustor that require additional cooling. Increasing the film cooling flow will solve this problem but results in excess cooling air for the regions that do not require additional cooling, thereby reducing engine performance. Accordingly, preferential cooling hole patterns have been developed wherein different hole diameters are used over the combustor so that additional cooling is provided only where needed.
One specific condition that can reduce cooling film effectiveness is the presence in the combustor liners of igniter towers that hold the igniters for initiating combustion in the combustor. The igniter towers disrupt the flow of cooling air over the combustor liners, thereby reducing the cooling film effectiveness behind them. Thus, regions of the liners immediately downstream of the igniter towers will be prone to a loss of cooling film effectiveness. This condition has been addressed by adding a set of radially oriented cooling holes to the nugget immediately downstream of the igniter towers as well as increasing the diameter of some of the axially oriented cooling holes in that nugget. However, because many different cooling hole sizes and patterns are employed, several tooling set-ups are required. This adds to the time and cost of manufacturing the liners.
Accordingly, there is a need for a combustor liner having a cooling scheme that optimizes cooling film effectiveness while minimizing tooling set-up requirements.